The present invention relates to a torque sensor for sensing a torque generated in a rotary shaft, and more particularly to a torque sensor which is improved in the sensitivity with a simple construction and in which the axial accuracies of component parts and axial errors in assembling these parts affect little influence to the torque sensing accuracy.
One of conventional torque sensors is described in U.S. Pat. No. 4,805,463. In the conventional torque sensor, an input shaft is coaxially and rotatably coupled with an output shaft. The overlapping portion of those shafts is surrounded by an aluminum tubular member, which is relatively short. The tubular member covering the overlapping portion of the shafts is axially movable with a relative motion of the input shaft to the output shaft. A coil is disposed around the tubular member. A self-induction electromotive force induced in the coil is measured. A relative turn (torque) of the input shaft to the output shaft is detected by using the result of the measurement. When the tubular member is axially moved, the self-inductance of the coil changes. Accordingly, a torque generated in the input and the output shafts can be detected by using the self-induction electromotive force of the coil.
However, a mechanism for converting the relative turn of the first shaft to the second shaft into an axial displacement of the tubular member is indispensable for the conventional torque sensor. This leads to complexity of the sensor structure and to reduction of the reliability of the sensor.
Further, only the axial movement of the aluminum tubular member is insufficient for causing a rapid change of the self-inductance of the coil. To increase the sensitivity of the sensor, it is necessary to increase the number of turns of the coil, for example. This leads to the defect that the sensor size is increased.
Also, there is a conventional power steering system having the following construction. In the power steering system, a torsion bar, which is elastically deformable in the direction of a twist of it, is incorporated into a steering system of a motor vehicle. A relative turn, proportional to a steering torque, is caused between a couple of first and second shafts, which are coupled with each other by the torsion bar. The steering torque is detected by measuring the relative turn. An auxiliary steering torque is generated in accordance with the detected torque. The auxiliary steering torque lessens the load to a driver. A torque sensor of the type in which such a relative turn is measured, is also known. In the torque sensor, an impedance of the coil is varied in accordance with a steering torque, and the torque is detected by measuring the impedance of the coil (see U.S. Pat. No. 4,996,890, for example).
However, in the above-mentioned torque sensor, the coil is fixed to the housing, and the member for varying the impedance of the coil is fixed to the shaft rotatable relative to the housing. Therefore, the axial assembling errors and the manufacturing errors greatly affect the sensing accuracy of the torque sensor. To obtain the satisfactory sensing accuracy, the assembling accuracy and the part accuracy must be high. This results in the defect in the cost of manufacture.
Further, one of the conventional torque sensors is disclosed in U.S. Pat. No. 4,356,732. In the torque sensor, two tubular members are arranged coaxially with each other so that one of the tubular members turns relative to the other in accordance with a torque generated in a rotary shaft. Long grooves and teeth are alternately formed in the outer surface of the inner tubular member. Cutouts are formed in the outer tubular member so that the overlapping areas of the cutouts and the grooves vary with a relative turn of the tubular members. A coil is disposed surrounding the outer tubular member. When the overlapping areas of the cutouts and the grooves vary with a relative turn of the tubular members, the impedance of the coil vary. Therefore, a torque generated in the rotary shaft can be detected by measuring the impedance of the coil.
Indeed, the conventional torque sensor detects a torque generated in the shaft by using the impedance change of the coil. However, in the torque sensor, the change of the impedance is not rapid. Therefore, the sensor sensitivity is not satisfactory.